WASHINGTON
— As the Senate moves toward a showdown over the so-called nuclear option, risks and rewards confront both Republicans and Democrats, whatever the outcome.

Both sides concede that the move to lower the threshold required to end a filibuster from 60 votes to a simple majority could shut down the Senate. But it's not clear for how long, with what consequences, and who would be blamed if the Senate's work grinds to a halt.

When almost half of federal employees stopped work at noon on Nov. 14, 1995, President Clinton blamed the Republican Congress. Most Americans believed him. The GOP lost seats in the 1996 elections, and House Speaker Newt Gingrich later resigned.

For Republicans, it's a test of whether they can move the president's nominees through a Senate they now control with a margin of five votes. The judicial impasse has become a defining issue for Senate majority leader Bill Frist, who is weighing a presidential run in 2008.

For Democrats, the challenge is to hold the line on nominees they deem unqualified, while avoiding the label "obstructionist." Former Senate minority leader Tom Daschle lost his seat in the 2004 election after outside groups poured millions into his state to promote that view.

"The Senate is about to enter its own cold war," says Jennifer Duffy, Senate analyst for the Cook Political Report. "Democrats have done a very good job of backing [Senate majority leader Bill] Frist into a corner and keeping him there."

At the same time, powerful interest groups in both camps are fueling the conflict with ad campaigns, petition drives, and rallies outside the US Senate. Both sides accuse the other of being driven by "extremist groups."

In a videotaped speech to a Christian conservative rally in Louisville, Ky., on Sunday, Senator Frist renewed his call for what Republicans now call the "constitutional option," should Democrats filibuster another judicial nominee. "Now if Senator Reid continues to obstruct the process, we will consider what opponents call the 'nuclear option.' Only in the United States Senate could it be considered a devastating option to allow a vote. Most places call that democracy," Frist said.

At a Monitor breakfast on Monday, Senator Democratic leader Harry Reid gave no signs of backing off the filibuster threat. For the first time, he also laid out plans for a post nuclear-option Senate. "I have always said we wanted to make sure that the Senate went forward, but we're going to do it on our own agenda," he said.

In recent days, Democrats have been quietly putting their own bills on the Senate calendar. Using an obscure Senate procedure called Rule XIV, they plan to move these bills onto the agenda if Republicans "pull the trigger" on the nuclear option. By tradition, it's the Senate majority leader who sets the Senate agenda.

"By invoking the nuclear option, they will have shattered the comity in the Senate, so Democrats won't be bound by the usual way of doing business," says Jim Manley, spokesman for Senate Democrats.

These bills include legislation to reduce the number of unintended pregnancies and abortions by increasing funding for family planning, giving more help to disabled veterans, renewing pay-as-you-go provisions to curb federal deficits, guaranteeing overtime pay for workers, and raising the minimum wage.

Despite two prominent defections, Republicans say they have the votes to win on the Senate floor, should the nuclear or constitutional option come to a vote. Some Democrats dispute that vote count. "It would already have come to a head if they had the votes," Senator Reid said.

Meanwhile, polls show tepid public support for the nuclear option. A recent private poll, reported by the Associated Press, signals that only 37 percent of Americans support the GOP plan to change the filibuster rules; 51 percent opposed a change.

At the same time, business groups are also signaling a rift on the issue. While the National Association of Manufacturers has thrown its support behind the Republicans on judges, the US Chamber of Commerce is urging compromise.

"I'm hopeful that both parties will find a way to get together and allow votes on at least a few of these [judges], so we avoid tying up the Senate," said Thomas Donohue, president of the US Chamber of Commerce on Friday. Should the Senate gridlock over the issue, it's unlikely that Congress will complete an energy bill, fix healthcare programs, expand free trade, resolve asbestos lawsuits, or advance anything else on the business agenda.

But supporters say the filibuster fight has come too far for Republicans to back down. And some say even a failed push for the judges could bolster Republicans' standing with their conservative base.

"Senator Frist has been patient beyond any reasonable level," says Sen. Kay Bailey Hutchison (R) of Texas, vice chair of the Republican Conference. "Even if he loses, he is going to keep his word."

Meanwhile, Democrats say that Frist's stance is being driven as much by outside groups and his own goals as by the dynamics of the Senate. "I'm afraid his presidential aspirations are getting in the way of his Senate leadership position. I hope he can overcome this," Reid said. Republicans charge that Democrats are being driven by their own array of outside interest groups.

Some on both sides hope a showdown, and its after-effects, can still be averted.